Antigen-specific soluble helper molecules are produced during major histocompatibility complex-disparate allograft priming. Genetic mapping studies with appropriate recombinant and mutant lines of mice have defined the antigen specificities of the soluble helper molecules described here as being directed against the H-2Dd molecules. The production of antigen-specific helper molecules is a relatively early event after H-2Dd region allograft priming. Supernatants containing the antigen-specific helper molecules have no, or at least undetectable levels of, interleukin-2 (IL-2). Also, from the draining lymph nodes, we have demonstrated a later phase of factor production near the time of graft rejection thath does contain nonspecific helper factors and IL-2. We have further demonstrated that supernatants obtained six days after allograft priming contain MHC-restricted factors that activate IL-2 production. In other studies we have demonstrated that monoclonal antibodies directed against the T-cell surface molecule, L3T4, block the generation of syngeneic cytolytic effectors against TNP-altered self. The mab-mediated inhibition results in the induction of active suppressor cells. The induction of suppressors can be overcome by a mixture of recombinant IL-2 and a second T-cell-derived factor. A T-cell hybridoma has been isolated that produces this second "helper factor." (LB)